Switching power supplies are ubiquitous. So many devices require DC power, and the power grid provides AC power as a source. Accordingly, the first step undertaken by many devices using electrical power is to accept AC power from a connection to the power grid and convert the AC power to the necessary DC level or levels for operation of the device. Power grid standards, e.g., IEC 61000-3-2 govern power quality and distortion limits based on the manner in which converters may draw power.
In high power converters, there are compelling reasons from a power usage standpoint to carefully control the power factor imposed by a converter. Drawing less than ideal power factors wastes significant amounts of energy, and might also violate the standards imposed on the draw taken by the converter and its requisite effect on the power grid. Active power factor correction (PFC) is widely applied to high power off-line converters to enforce unity power factor operation. A typical implementation is a two-stage converter system, in which the first stage regulates the (input) current waveform from the power grid and the second stage adds the degrees of freedom necessary to regulate both the input current and output voltage simultaneously. The extra stage adds cost and reduces efficiency. Active PFC results in sufficient efficiency in high power applications to offset the expense of the extra active PFC stage. The expense is sufficient, and the power draw high enough, to demand that near unity power factor and a low distortion input be achieved.
Conventional low power converters, on the other hand, generally exhibit low power factors as the amount of power draw is small enough that impact on the power grid has been ignored. Low device cost is the prevailing concern in consumer markets, where manufacturer margins are small. Extra cost from an active PFC process becomes a problem, so the methods are rarely used at low power levels. Converters are kept inexpensive, and the result is the imposition of high distortion and low power factor on the utility grid supplying power. In an inexpensive, low power factor, high distortion converter, a filter capacitor sufficiently large to account for the possible fluctuations indirectly imposed by the distortion and low power factor is required.
It is well known that single-stage PFC versions can be created, usually at some sacrifice in quality. For example, a buck converter can be used as a PFC front end, but only if the user is satisfied with limited regulation range: the converter cannot regulate input current during times when the input voltage is below the desired output. Quality suffers, but not in a way that is readily quantified.
The typically implemented active PFC circuit achieves high-quality performance. It is recognized, though, that power quality standards do not require such performance. See, e.g., O. Garcia et al., “Single Phase Power Factor Correction: a Survey,” IEEE Trans. Power Electronics, vol. 18, no. 3, pp. 749-755, May 2004. The general presumption in implementing a power factor correction is that near unity power factor will be implemented. This may be due to the all-or-nothing approach in the art, as higher power applications that most often use PFC will be driven towards ideal correction because of the cost savings in power draw.
It is useful in understanding the description of the invention below to first consider a typical single-phase power conversion application, and recognize two extremes, one in which there is a conventional PFC correction to achieve unity or near unity power factor, but with a resultant high double frequency power term, and another where a near constant power is drawn by a converter, but with an unacceptable power factor. Neither design extreme, ideal power factor nor constant input power, is likely to be optimal in terms of power loss, filter performance, or cost. The ideal power factor case may need a two-stage converter, and requires a large filter capacitor. The constant power case imposes a high loss penalty within the energy source. Generally in the art, PFC has been implemented in higher power applications to achieve unity power factor. In low power applications, on the other hand, PFC is typically not used and power factors are permitted to approach 0.5, and a filter is exclusively used to account for variations in the drawn power without any active control.